A black toenail sounds like the result of some kind of injury. Toenails are quite strong, so although trauma to your toes may not damage the nail itself, it may damage the nail bed. This is basically an injury under the nail - blood therefore gets trapped under the toe nail, which is what gives the nail its black appearance. Such injuries are usually minor injuries, and require no treatment except for a few days of rest. If you visit a doctor immediately, it may be possible to drain the blood from under the toenail, but this is not really necessary. As the nail grows out, the discoloration will go away. However, if the toe is extremely painful, if it takes too long to heal, or if the nail does not seem to be growing properly, you should show it to your doctor.
This can sometimes happen even without an actual injury - this is often the case with people who do a lot of running in shoes that are too tight for them. It can however happen to anyone wearing shoes that are ill fitting. What happens is that your toe and toenail keep bumping against the front of the shoe as you walk or run, or perhaps the front of the shoe presses down too hard on the toenail. As with an injury, this may cause blood to pool under a toenail, and sometimes a blister may also form. With rest, the problem usually takes care of itself, but if this happens regularly, you need to look for a more permanent solution. The most logical step to take is to change your shoes for a pair that fits you better - make sure that your shoes are not too tight, else they will press down on your toes, and make sure that they are not too loose, else there will be space for your toes to bump around as you walk.
answered by M W